--- a/includes/common.inc	2007-11-30 00:53:59.000000000 +0200
+++ b/includes/common.inc	2007-11-30 00:42:57.000000000 +0200
@@ -1877,7 +1877,9 @@
 function page_set_cache() {
   global $user, $base_root;
 
-  if (!$user->uid && $_SERVER['REQUEST_METHOD'] == 'GET' && count(drupal_get_messages(NULL, FALSE)) == 0) {
+  if (!$user->uid && $_SERVER['REQUEST_METHOD'] == 'GET' && count(drupal_get_messages(NULL, FALSE)) == 0 && 
+      extension_loaded('zlib') && !ini_get('zlib.output_compression') && ini_get('output_handler') != 'ob_gzhandler' &&
+      ((version_compare(phpversion(), '5.0', '>=') && ob_get_length() == false) or ob_get_length() === false)) {
     // This will fail in some cases, see page_get_cache() for the explanation.
     if ($data = ob_get_contents()) {
       $cache = TRUE;